The present invention relates generally to automatic frequency control circuits and in particular to a variable frequency oscillator which is free from disadvantages resulting from supply voltage fluctuations and nonuniform circuit parameters. The invention is particularly suitable for automatic phase control (APC) of color television receivers, and specifically suited for production in integral circuit configuration.
In prior art variable frequency oscillators, two phase-shifted signals are generated within a single loop of oscillation including an amplifier and a bandpass filter. A vector sum of the two signals is applied to the amplifier to generate oscillation. The input to the amplifier is controlled in phase by varying the amplitude of the two phase-shifted signals relative to the other. The prior art oscillators, however, have drawbacks in that possible nonuniformity in circuit parameters that determine the relative phase shifts of the two signals will result in an output frequency differing from the intended value and possible supply voltage fluctuations will cause the amplitude of the signals combined to drift indefinitely. Therefore, it is difficult to precisely maintain the output frequency at a constant value.